Job Level 1?

[Nestama]

Unless you're BLM, where every offensive spell you get changes your rotation

Sort of... Fire III is only ever used to give you instant Astral II, likewise with Blizzard III until you get Magick and Mend III. You then get a single target rotation you stick with from level 40 to level 60. Firestarter being a nice trait and Thunder III completely replacing Thunder II.

Your AoE rotation remains the same from the moment you learn Fire II until you learn Flare (which is a closing spell).